1. Field of the Invention
The present invention relates to a fabrication method of a printed wiring board and more particularly, to a fabrication method of a high-density printed wiring board having fine-pitch soldering pads.
2. Description of the Prior Art
Conventionally, to prevent solder bridges from being formed on a high-density printed wiring board having fine-pitch soldering pads during a soldering process, melted solder has been stopped flowing by blocking dams made of solder resist whose surfaces are higher than those of the soldering pads. In general, the blocking dams have been constructed by either (a) applying solder resist ink on one surface or both surfaces of the wiring board through a screen printing process, then to cure or harden the ink thus printed, or by (b) coating photosensitive solder resist on one surface or both surfaces of the wiring board, selectively exposing the solder resist film thus formed using a mask, then develop the solder resist film thus exposed.
With such conventional method, in case the thickness of the pitches of the soldering pads being 500 .mu.m or less, however, there is a problem in positional adjustment or matching in (a) the printing process of the solder resist ink or in (b) the exposure process of the photosensitive solder resist film becomes very difficult and as a result, the solder blocking dams cannot be provided with sufficient precision.
To solve this problem, a fabrication method of a printed wiring board as shown in FIGS. 1A to 1D has been developed, which is disclosed in the Japanese Non-Examined Patent Publication No. 3-268479.
Though the printed wiring board has a plurality of soldering pads 21a, lands 21b, wiring lines 21c and through holes 22, to simply description, only one of the through holes 22, two of the lands 21b corresponding to the one hole 22, three of the wiring lines 21c adjacent to the through hole 22 are shown in FIGS. 1A, 1B, 1C and 1D.
With this conventional fabrication method, first, a copperclad laminate 23 composed of a sheet-like insulating base material and first and second copper foils fixed to each surface of the base material is prepared. The through holes 22 are formed through the laminate 23 and conductive metal such as copper is plated on the inner walls of the respective through holes 22 to interconnect the first and second copper foils with each other.
Next, etching resist films 25 having given circuit patterns are respectively formed on the first and second copper foils, and both of the copper foils are selectively etched using the patterned etching resist films 25 as masks to form the given circuit patterns on each surface of the base material, respectively.
Thus, as shown in FIG. 1A, the soldering pads 21a, lands 21b and wiring lines 21c are formed on a first surface of the base material and the lands 21b and wiring lines 21c are formed on a second surface thereof. The lands 21b on the first and second surfaces are interconnected with each other through the corresponding plated through holes 22.
Without removing the etching resist films 25, solder resist material is coated to cover the respective copper foils so that solder resist films 24a and 24b are formed on the first and second surfaces of the base material, respectively.
Thereafter, the surface areas of the solder resist films 24a and 24b are removed by grinding using a belt grinding machine until the tops of the soldering pads, and 21b and the wiring lines 21c are exposed from the solder resist films 24a and 24b, respectively. The etching resist films 25 on both copper foils are removed during this grinding process.
Thus, as shown in FIG. 1B, the surfaces of the unremoved solder resist film 24a, and soldering pads 21a, lands 21b as well as the wiring lines 21c are the same in height thereby forming a flat plane over the first surface of the base material. Similarly, the surfaces of the unremoved solder resistant film 24b and soldering pads 21b and the wiring lines 21c are also of the same thereby forming another even plane over the second surface of the base material. At this time, the etching resist films 25 in the respective through holes 22 are not removed.
Next, surface areas of the soldering pads 21a, lands 21b and wiring lines 21c are selectively etched to given depths so that solder resist dams 24a and 24b are produced on the first and second surfaces of the base material, respectively, as shown in FIG. 1C. Subsequently, the etching resist films 25 remaining in the through holes 22 are removed.
Thus, the structure as shown in FIG. 1D is obtained, in which the blocking dams made of the remaining solder resist films 24a and 24b are formed adjacent to the soldering pads 21a, lands 21b and wiring lines 21c, respectively.
Subsequently, though not shown, melted solder is poured on soldering pads 21a and lands 21b to form solder films thereon, respectively, resulting in a printed wiring board.
Desired electronic components or devices are placed on the soldering pads 21a and/or the lands 21b to be mounted by soldering in an assembly process of the printed wiring board.
Due to the solder resist dams 24a and 24b, even if the electronic components or devices are soldered to be mounted on the soldering pads 21a and/or the lands 21b at narrow pitches such as 500 .mu.m or less, solder bridges are prevented from being formed.
With the conventional fabrication method shown in FIGS. 1A, 1B, 1C and 1D however, there are the following problems:
(1) Since soldering pads 21a, lands 21b and wiring lines 21c are exposed from the solder resist films 24a and 24b through the grinding process, thicknesses of pads 21a, lands 21b and lines 21c are difficult to be controlled precisely. As a result, satisfactory soldering reliability cannot be ensured.
(2) Since grinding process is needed, fabrication cost increases.
(3) Because the patterned first and second copper foils of the copper clad laminate are entirely exposed through the grinding process, solder resist is necessary to be coated again to cover both copper foils in their entirety except for areas not to be soldered.